* Re: _stp_gettimeofday_ns(void) in stp VS ktime_get_real() in Kernel
[not found] <9f49f7820912131824p5f6589f5n85f32889577cd067@mail.gmail.com>
@ 2009-12-14 2:35 ` Frank Ch. Eigler
0 siblings, 0 replies; only message in thread
From: Frank Ch. Eigler @ 2009-12-14 2:35 UTC (permalink / raw)
To: 曾珊; +Cc: systemtap
Hi -
> I am now using the following script to compute the executing of the
> netif_receive_skb funcion: [...]
> and the result is some thing like :
> 0 qemu-system-x86(3822):->netif_receive_skb
> 4373 qemu-system-x86(3822):<-netif_receive_skb
>
> *Question 1:
> *Can I use 4373 which is 0 subtract from 4373 to describe the executing time
> of the function netif_receive_skb?*
Yes, that is the intent.
> **Question 2:*
> since the way in which thread_indent function to get the time is through
> _stp_gettimeofday_ns(void) function and in kernel skb->tstamp is set by
> the function ktime_get_real() , my question is whether the principles of the
> two are the same? or whether they are comparable?
High-precision timing is a complicated subject. Different generations
of kernels have a variety of different time measurement widgets,
useful in only certain contexts each. The systemtap runtime has had to
have a separate get_cycles()-based estimator that it can use in atomic
contexts. They are not directly comparable.
However, you can calibrate the measurements for your own purposes,
calling a variety of time sources (get_cycles(), gettimeofday_*,
$skb->tstamp, your own embedded-c functions to fetch ktime_get_real()),
and see to what extent they all "line up".
- FChE
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